can i clarify that why isn't "hay tantas cosas por hacer" correct instead of "Hay tantas cosas que hacer"? It would be useful if you could direct me to the relevant notes, if there is. 

Another question is that, why is "todos los vecinos de la comunidad" using tuvimos instead of tuvieron when they had a meeting?

One last one is to clarify is "representar" + preposition A, a fixed expression to express on behalf of (someone) and that in this sentence, it is not personal A because we are referring to a general group of people? or the group of neighbors are not considered as general but specific persons?

Your replies are much appreciated. thanks.